The Response of Two-Dimensional Granular Superconducting Films to Light
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THE RESPONSE OF TWO-DIMENSIONAL GRANULAR SUPERCONDUCTING FILMS TO LIGHT J. C. CULBERTSON, U. STROM, AND S. A. WOLF Naval Research Laboratory, Washington, D.C. ABSTRACT We have examined the photoresponse of two-dimensional granular superconducting films of high Tc superconductors (Y-Ba-Cu-O). We see evidence for the existence of a twodimensional Kosterlitz-Thouless transition. The photoresponse and resistance have been measured as a functions of temperature, excitation power, bias current, and photon energy. The response to photons having energies above and below the superconducting energy gap(s) has been measured. Photoresponse features have been observed that are not explainable in terms of a conventional phonon bolometric mechanism. Comparison will be made with similar observations made on low temperature superconducting films (NbN-BN). INTRODUCTION Photons can interact with superconducting films in a variety of ways that result in a change in the resistance of the film. Photons that have energies above the superconducting energy gap can, upon absorption, generate nonequilibrium distributions of particles (e.g. quasiparticles and magnetic flux vortices); such distributions can be described by T* models1 where T* > Tlattice. Photons that have energies near and below the superconducting energy gap can directly modulate the intergrain Josephson currents. Whether the process is nonequilibrium or direct, if the absorption of photon energy is involved, there will be an associated and generally slower phonon-bolometric signal. We have conducted our photoresponse studies on superconducting films that are granular. SAMPLE CHARACTERIZATION
The high temperature superconductor measurements discussed here were performed on a 2500 A thick film of YBa 2 . 1Cu 3.4 OT 7 8 with a 2 mm wide current path. This film was grown on an MgO substrate using magnetron sputtering. All electrical measurements were performed using a four point probe technique. The film is granular; a measurement of resistance vs temperature shows that the transition of the superconducting grains is at T cg = 82 K, but the film does not reach a zero resistance until the temperature drops to about Tc = 11 K; the resistance above the 82 K transition temperature peaks and then decreases with increasing temperature - showing an activated conductivity. One way in which a superconducting film can reach a zero resistance state is through a two-dimensional Kosterlitz-Thouless transition2,3 ; the Kosterlitz-Thouless transition in thin film superconductors is a transition involving the binding and unbinding of magnetic flux vortices to antivortices; unbound vortices driven by external forces will move across the current path thus giving the film a nonzero resistance, but if vortices are bound to antivortices and the separation is small compared to the inhomogeneity of the external fields then the net force on such a pair is zero and the film will have zero resistance. Measurements 4 of the voltage - current relations for temperatures around 11 K show a power law behavio
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